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Energetic particles (EPs) from fusion reactions and auxiliary heating can drive symmetry-breaking shear Alfvén wave (SAW) instabilities, including AEs and EPMs, via resonant wave-particle interactions, and understanding their nonlinear evolution is critical for understanding the alpha particle confinement in next-generation devices like ITER and CFEDR.
In our previous work [1], nonlinear simulations of Alfvénic instabilities in EAST ICRH experiments were carried out using the hybrid code MEGA [2]. The resonant excitation mechanisms of the dominant n=2 TAE-like EPM were elucidated, and nonlinear analysis revealed frequency bifurcation into high-frequency TAE and low-frequency BAE-like branches via upward and downward chirping, along with secular EP transport via phase-locking with the downward-chirping branch. However, the detailed mechanisms of the nonlinear frequency chirping remain to be fully understood.
To elucidate the underlying mechanisms, dedicated phase-space diagnostics [3] are employed in the present work to further investigate the nonlinear EPM evolution, based on the reference scenario studied in Ref. [1]. The results demonstrate that the nonlinear evolution is dominated not by a fixed cohort of initial resonant particles, but by the exchange of the resonant populations via self-consistent continuous trapping and de-trapping. This is evidenced by three representative wave-particle interaction behaviors: (i) initially resonant particles that become non-resonant and de-trapped; (ii) particles that remain trapped throughout; and (iii) initially non-resonant particles that become resonant during the nonlinear stage. In addition, the mode frequency chirping rate exhibits a linear scaling with the instantaneous mode amplitude, as predicted by the general theory [4,5]. Collectively, these results substantially advance the understanding of nonlinear EPM evolution [6], providing an intuitive physical picture fully consistent with the theory of nonadiabatic nonlinear evolution.
[1] Su P. et al 2026 Nucl. Fusion 66 022002
[2] Todo Y. et al 2015 Nucl. Fusion 55 073020
[3] Todo Y. et al 2021 Plasma Phys. Control. Fusion 63 075018
[4] Chen L. and Zonca F. 2016 Rev. Mod. Phys. 88 015008
[5] Zonca F. et al 2015 New J. Phys. 17 013052
[6] Su P. et al 2026 Nucl. Fusion (submitted)